Those of ordinary skill in the art will appreciate that the oil industry often uses three-dimensional (3D) visualization to showcase its exploitation of the latest high-tech developments. (As used herein, the term “visualization” is intended to encompass a process involving the computer processing, transformation, and visual/graphical display of data to facilitate its interpretation.) Visualization has become a well-established planning and analytical tool for the geological and geophysical (G&G) segment of the industry. Benefits extend beyond technical issues, as communal visualization has promoted multi-disciplinary discussion and created opportunities to bring people together and improve the dynamics of E&P teams by providing clarity in the face of the ever increasing amount of data that forms the modern well construction process.
Similar success is being achieved in drilling. Early applications focused on well placement in complex reservoirs, and directional drilling to control well tortuosity and avoid collisions on multi-well platforms. More recent applications use 3D visualization to address drilling problems and link drilling operational data to earth models. Countless other drilling prospects should exist, especially considering that “making hole” occurs out of sight, miles below the earth's surface.
In the prior art, downhole visualization has focused principally on the trajectory of the borehole, particularly with the ever-increasing popularity of directional drilling. Knowing the precise location of the borehole at all points along its length is critical to ensure that the drilling operation succeeds ultimately in the borehole arriving at the desired production region.
Downhole video is a proven telepositioning method for mechanical inspection, fishing operations, and problem investigation in a wellbore. Typical problems include damaged liners, casing holes, corrosion, sand/fluid entry, and wellbore plugging. Downhole cameras use fiber-optic technology to produce black and white images at working temperatures up to 350° F.
Unfortunately, downhole video cannot be used during drilling, since (a) nearly all drilling fluids are opaque; (b) normal drilling operations would have to be suspended; and (c) the drill string would interfere with camera operations.
Videos taken of simulated wellbores in the laboratory, despite temperature and pressure shortcomings, have contributed significantly to the industry's understanding of downhole behavior, especially hole cleaning and barite sag. Remarkable footage captured through transparent, inclined flow loops have documented the impact of different parameters on hole-cleaning efficiency, including hole angle, annular velocity, pipe eccentricity and rotation, low-shear-rate viscosity, flow regime, and avalanching cuttings beds. Video has also helped validate the field success with drilling horizontal wells at some sites with rheologically engineered bio-polymer drill-in fluids, a hole-cleaning concept that was contrary to industry thinking at the time. Additionally, laboratory studies based on extensive video imaging helped convince the industry that barite sag was primarily a dynamic settling problem and not the static problem as previously thought.
Nevertheless, as noted, video examination of actual wellbores is not a practical alternative, meaning that other means must be employed to analyze dynamic parameters of the interior of a wellbore.
To address this problem, wellbore hydraulics applications have been developed for simulating the dynamics of fluid flow within wellbores based on known or modeled data about the well. Wellbore hydraulics applications have had a long history of steady improvement, and were adequate, for the most part, until synthetic-based muds became the drilling fluids of choice for deepwater projects in the early 1990s. The deepwater environment is characterized by low fracture gradients, narrow operating windows, and low temperatures approaching 40° F. Density and rheological properties of synthetic-based fluids are sensitive to temperature and pressure, and these premium fluids were extraordinarily expensive at the time. For these reasons, lost circulation was considered (and often still is) a particularly critical problem.
Advanced software has emerged that considers, among other things, the effects of temperature and pressure on density and rheology. Numerous commercially-available examples of such hydraulics analysis applications are known in the art. An interesting aspect of such programs is that the modules created for calculating equivalent static densities (ESDs) are based on numerical integration of short wellbore segments. This approach has set the stage for using techniques involving finite difference analysis for other calculations.
Generally speaking, hydraulics applications function to take a number of dynamic, depth-varying parameters for a wellbore (and drillstring) as inputs to provide as an output one or more indicators of well performance and behavior.
Outside of the oil industry, video has long been a mainstay to view objects not easily accessible. The medical field is perhaps the best known, and the colonoscopy is an excellent analogy to downhole video technology. A colonoscopy allows doctors to detect colorectal polyps (growths) and cancers. In this rather unpleasant but very important procedure, a colonoscope is inserted and slowly guided up through the patient's colon. A tiny camera in this long, flexible, lighted tube transmits images that allow the doctor to examine the lining of the colon on a video monitor. If an abnormality is detected, the doctor can remove it or take tissue samples using tiny instruments passed through the scope.
Recent technological advancements have made it possible to perform a non-invasive procedure called a “virtual” colonoscopy. The virtual colonoscopy provides computer generated images that look similar to those seen by the traditional version. The process involves performing a spiral (or helical) computer-aided tomography (CAT) scan, wherein a rotating x-ray machine follows a spiral path around the body. A high-powered computer uses the x-ray data to create detailed cross-sectional pictures of the body. The high-resolution, 2D pictures are then assembled like slices in a loaf of bread to construct a detailed, 3D image of the colon lining suitable for thorough analysis by the doctor.
Virtual images created for medical use invariably are based on measured data. Unfortunately, detailed data required along a well path cannot be measured with current technology. The alternative is to simulate the downhole drilling process with appropriate models. Logically, the accuracy of the models is important.
Accuracy is a serious issue regarding downhole simulation. For present-day industry hydraulics programs, numerical validation typically is preferably achieved by comparing results to annular-pressure-while-drilling (APWD) measurements and pump pressures, unless special instrumentation has otherwise been added. Annular fluid behavior clearly has received the most attention. Most advanced hydraulics programs have evolved to the point where it is not uncommon for calculated equivalent circulating densities (ECDs) to consistently be within 0.1 lb/gal of APWD measurements.
By definition, however, 3D visualization exposes the entire simulation to scrutiny. As such, this causes assumed, uncertain, and ignored parameters to inadvertently stand out. Drill-string eccentricity, a non-trivial calculation, is a good example. Most industry hydraulics programs ignore eccentricity in directional wells, while others assume a constant value. For visualization to be credible, a reasonable estimate of drill-string eccentricity throughout the entire well profile should be provided.
Still other parameters are so complex and unknown that significant additional development is required. Examples include the annular velocity profiles across bottomhole assemblies, impact of downhole tools on cuttings and barite bed movement, and orbit of the drill string around the borehole center when side loads are high. Clearly, models can be improved with additional downhole data and laboratory studies.